Vehicle systems may include multiple coolant loops for circulating coolant through distinct sets of engine components. The coolant flow may absorb heat from some components (thereby expediting cooling of those components) and transfer the heat to other components (thereby expediting heating of those components). For example, a high temperature coolant loop may circulate coolant through an engine to absorb waste engine heat. The coolant may also receive heat rejected from one or more of an EGR cooler, an exhaust manifold cooler, a turbocharger cooler, and a transmission oil cooler. Heat from the heated coolant may be transferred to a heater core (for heating a vehicle cabin), and/or dissipated to the atmosphere upon passage through a radiator including a fan. As another example, a low temperature coolant loop may circulate coolant through a charge air cooler. When required (such as when cabin air conditioning is requested), coolant in the low temperature loop may be additionally pumped through the condenser of an air conditioning (AC) system to absorb heat rejected at the condenser by a refrigerant of the AC system. Heat from the heated coolant may be dissipated to the atmosphere upon passage through another radiator including a fan. One example of such a vehicle coolant system is shown by Ulrey et al. in US20150047374. Another example coolant system is shown by Isermeyer et al. in US20150040874. Therein a heat exchanger enables heat exchange between a charge air cooling coolant circuit and a refrigerant circuit of the condenser.
However, the inventors herein have identified potential issues with such coolant systems. As one example, in vehicle systems where the AC system is configured to provide maximum cooling at all times, fuel economy may be lost due to the need to operate the coolant pump continuously. In addition, the pump may suffer from excess wear resulting in warranty issues. Even during steady-state conditions, the pump output may be higher than required. On the other hand, if the pump is selectively deactivated when there is no AC demand, then there may be delays in delivering cabin cooling when AC is subsequently demanded. In particular, there may be a delay incurred in operating the pump and providing the required coolant flow through the AC condenser to meet the cooling demand.
Another potential issue is the position of the AC system with relation to other under-hood components. In embodiments where the AC condenser has a dedicated refrigerant circuit, such as in the approach of '874, the AC condenser is positioned at the front end of the vehicle, in front of other under-hood components, allowing a larger portion of vehicle cooling air to be directed to the condenser. However in approaches where the AC condenser shares coolant with other components, the AC condenser may need to be moved further away from the front end of the vehicle, for example to make space for the radiator of a high temperature coolant circuit. At the new location, the condenser may receive a smaller portion of the vehicle cooling air, thereby increasing the reliance on the coolant pump. Also, the under-hood temperature may vary from a steady-state temperature, requiring additional coolant to be pumped to carry away the under-hood heat and operate the AC system at equilibrium.
Still another potential issue in embodiments where the coolant is shared between the condenser and other engine components (such as a charge air cooler) is that during conditions when both AC demand is high and engine cooling demand is high, it may not be possible to meet both demands. As a result, one or both of the cooling demands may remain compromised.
In one example, the above issues may be addressed by a method for operating a vehicle air conditioning system, comprising: in response to each of a cabin cooling demand and an engine cooling demand being higher than a threshold, adjusting coolant flow through each of an air-conditioning (AC) condenser and a charge air cooler (CAC) of a coolant circuit, in parallel, to meet the engine cooling demand and cabin cooling demand, the coolant flow adjusted based on an AC head pressure and further based on CAC outlet temperature. In this way, coolant flow may be better apportioned between components of the coolant circuit based on demand and performance, thereby enabling the cooling demands of all the components to be met.
As an example, each of a condenser of an AC system and a CAC may be coupled to distinct branches of a coolant circuit downstream of a proportioning valve, coolant directed into the circuit via a coolant pump. The condenser may be further coupled to a refrigerant circuit of the AC system, while the coolant circuit may be further coupled to an oil circuit of a transmission at a transmission oil cooler. The AC condenser may be positioned towards a rear end of the under-hood area. During vehicle operation, as driver demand and cabin cooling demand changes, the apportioning of coolant flow to each branch may be varied. For example, when cabin cooling is demanded, a desired coolant temperature is determined. Then, by referring a 2D map or model that maps a relationship between the coolant flow rate, the desired coolant temperature, and an AC head pressure, taking into account parasitic losses, a target coolant flow rate through the AC condenser may be determined (as the point of minima of the asymptote of the 2D map). In particular, there may be a coolant flow rate above which the change in coolant temperature is not significant due to an increase in parasitic losses at the CAC. This coolant flow rate may be set as the desired coolant flow rate through the AC loop. In addition, a corresponding reference AC head pressure may be determined. The desired coolant flow rate is then provided in a feed-forward manner via a combination of pump output adjustment and proportioning valve adjustment while taking into account constraints of other loops and other system components. Based on feedback regarding an error between the actual AC head pressure and the reference AC head pressure, the flow may be further adjusted. For example, if the AC is working harder than expected, coolant flow through the AC loop may be increased by increasing the opening of the valve towards the AC loop. The coolant flow rate through the AC condenser may be continually varied as AC operating conditions change and affect AC head pressure. During conditions when no cabin cooling is demanded, a minimum coolant flow rate may be determined to provide a coolant temperature that is determined based on the ambient temperature. During conditions when both cabin cooling demand and engine cooling demand is requested, and both demands are saturated, a calibrated fixed, calibrated coolant flow rate may be provided through both the AC condenser and the CAC.
In this way, coolant flow may be apportioned to different components to meet their cooling demands in a fuel efficient manner. By maintaining a minimum coolant flow rate through the AC condenser even when no cabin cooling is demanded, delays in delivering cabin cooling when cabin cooling is subsequently demanded are reduced. In particular, responsive to a sudden increase in cabin cooling demand, the pump output and the proportioning valve position may be varied to rapidly provide the required coolant flow through the AC condenser to meet the cooling demand. Further, during conditions when both cabin cooling demand and engine cooling demand is elevated, by providing a fixed coolant flow ratio through the distinct components, it may be possible to meet both demands without compromising any one demand. The technical effect of sharing coolant between various components having distinct cooling demands is that the need for additional cooling components, such as radiators and fans is reduced. This allows the AC components to be placed in a rearward location of the under-hood area which reduces warranty issues without compromising cooling abilities. Overall, engine cooling performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.